This invention relates to a location system for determining the position of an object in space and particularly to a location system which uses Ultra-Wideband (UWB) radio.
A typical location system determines the location of objects in three-dimensional space in relation to one or more reference points, whose location is known. If the relative location of an object is known in relation to sufficient reference points then the absolute location of the object may be determined.
FIG. 1 illustrates an array of reference points (A) having a known location and an object (B) whose location is unknown. The relative location of the object in relation to a reference point can be expressed in a multitude of ways. For example, the distance or bearing of the object from the reference point might be used. Once the relative location of the object is known in relation to a single reference point, the possible position of the object is restricted to being within a particular area of space. For example, in a three-dimensional system, if the object is located a distance X from a reference point, then the object must be located on the perimeter of a sphere centred at the reference point and having a radius X.
The possible location of the object can be further restricted by determining the relative location of the object in relation to other reference points. Although the relative location of the object in relation to a minimum number of reference points is required to determine the absolute location of an object (e.g. four for locating an object in three-dimensional space), in practice the absolute location of an object can often be determined using fewer reference points and discounting potential locations that are unrealistic.
The absolute location of an object may be determined in various ways depending on the known relationship between the object and the reference points. For example, well-known methods include triangulation (using relative distances and bearings) and trilateration (using relative distances).
One way to establish the relative location of the object is to transmit a signal between the object and a receiving device having a known location. For example, a suitable system can be based on “ultrawideband” (UWB) signals, which are short pulses of radiofrequency energy. In a location system using UWB technology, the distances and/or bearings are measured by sending UWB pulses, typically on the order of one nanosecond long, between the object to be located (or a marker attached to it) and the receiving device.
The distance between an object and a receiving device can be calculated by measuring the times-of-flight of the UWB pulses between the object and the receiving device. UWB signals are known to travel at the speed of light, therefore the distance between the object and the receiving device can be calculated from the time taken by a UWB pulse to travel between them. In order that the time-of-flight can be calculated, the object and the receiving device must have synchronised clock signals. However, the clocks of the object and the receiving device are unlikely to be exactly synchronised. Therefore, as the times involved are usually extremely short, any errors in the measurement of the time-of-flight will lead to the calculated distance being incorrect. As an alternative, the differences between the times-of-flight of the UWB pulses received by two or more receiving devices can be determined, enabling differential distances (‘pseudo-ranges’) between the object and those devices to be calculated. This method does not require the object and the receiving devices to be synchronised, although the receiving devices must be synchronised with each other. It is not possible to determine the absolute distances involved in this way.
By detecting UWB radio pulses sent from one point using an array of detectors (typically separated by at most a few wavelengths of the centre frequency of the UWB signal) placed at another and measuring the relative properties of the signals received at each antenna (such as time-of-arrival or signal phase) it is possible to determine the bearing from which the signal was received and therefore the bearing of the object from the receiving device.
In indoor or cluttered environments, UWB location systems have significant advantages in accuracy over other forms of radiolocation system, which operate on similar principles but use much longer radio pulses or even continuous signals. If longer radio signals are employed, reflections of the signals from surfaces within the environment tend to disrupt measurement of received signal properties (say, the time at which the peak of the signal is detected) introducing errors into the measured distances, pseudo-ranges or bearings. In contrast, a UWB radio pulse is so short that the entire pulse can typically be processed by a receiver in isolation (and hence with good accuracy) before signal reflections arrive at the receiver, because those reflections must travel a longer path than the direct signal.
The theory of one UWB ranging system suitable for use in a location system is described in the paper “Ranging in a Dense Multipath Environment Using an UWB Radio Link”, J-Y Lee and R. A Scholtz, IEEE Journal on Selected Areas in Communications, Vol. 20, No. 9, December 2002. A UWB signal is sent from one device to another and a signal is sent in response. This arrangement permits the round-trip-time of the signal from one device to another and back again to be determined and the distance between the devices can then be found by multiplying the round-trip-time by the speed of light and dividing by two. By repeating this process multiple times, with the device to be located communicating sequentially or simultaneously with several other devices placed at known points in space, the position of the first device can be determined. Typically, the UWB signals can also be coded to transfer data from one device to another. This data might include the identity of the device sending the signal, the identity of the device for which the signal was intended, and commands to modify the behaviour of the interacting devices, e.g. the rate at which each device transmits location-determination messages.
A disadvantage of this arrangement is that the object to be located (or the marker placed on it) must be capable of both transmitting and receiving UWB signals. UWB transmitters are typically quite simple devices, but UWB receivers are much more complex, and tend to be expensive and relatively power hungry (particularly when compared to traditional radio receivers). Therefore, the requirement for UWB-receive capability at the object to be located may well increase the cost of the system. Furthermore, because location systems are often used to determine the positions in space of mobile objects, which tend to have only battery power (rather than mains power) available to them, the additional power requirements of the UWB receiver at the mobile device may necessitate more frequent battery replacement or recharging, or a larger capacity battery, than would otherwise be desirable.
Systems in which the mobile object has the capability to both transmit and receive UWB signals are more flexible than those in which the mobile object only incorporates a UWB transmitter, because data can be conveyed to the mobile device over the UWB channel. However, systems of the first type are likely to be more expensive and power hungry than those of the second type, because they incorporate the added complexity of the UWB receiver.
Another implementation of a UWB location system is described in the paper “Commercialization of an Ultra Wideband Precision Asset Location System”, R. J. Fontana, E. Richley, J. Barney, Proceedings of the 2003 IEEE Conference on Ultra Wideband Systems and Technologies, November 2003, Reston, Va. In this system, a tag capable of transmitting UWB signals is attached to the object to be tracked. The UWB signals emitted by the tag are detected by a set of synchronised UWB receivers placed at known points in the environment, which determine pseudoranges to the tag and can then calculate its 3D position. In this arrangement, the mobile tags use very little power, because they simply transmit a train of UWB pulses which encode the tag's unique identifier and then enter a low-power state until the next time they must transmit. The interval between UWB pulse train emissions is fixed, but typically might be one second.
Other UWB technologies exist which do not support coding of the emitted pulse train signal with identity information. The transmitted signal can be sent on one of a number of independent channels and receivers can be tuned to one of those channels. The receivers then ignore signals received on other channels. However, the number of distinct channels is very small (perhaps only ten or twenty) and therefore the channels can not be used directly in lieu of an identity in a location system for tracking very many objects. These technologies are unable, on their own, to support such a location system, because it would be impossible to determine which signals were transmitted from which device.
A further consideration in the use of UWB location systems is the regulatory environment surrounding UWB radio technology. At present, in the USA, UWB systems can be certified and used under two sets of rules, one applicable to systems which are to be used only indoors (FCC Rules, Section 15.517), and one applicable to ‘hand-held’ devices which may be used indoors or outdoors (FCC Rules, Section 15.518). Indoor-only systems are offered slightly relaxed emissions limits, because they can rely on some attenuation from building structure to minimise potential interference to other radio systems.
Each set of rules specifies restrictions to which UWB transmitters must adhere if they are to gain certification. For indoor systems, the technical design of the UWB transmitter must be such that it is incapable of being used outdoors. The rules state that having the UWB transmitter be powered from the mains supply is sufficient to demonstrate this. For ‘hand-held’ systems, the UWB transmitter must cease operation within ten seconds if it does not receive an indication that an associated receiver has detected its signal. Furthermore, ‘hand-held’ systems may not make use of fixed infrastructure involving UWB transmitters.
Bidirectional UWB systems such as the first described above could in principle be designed to meet the ‘hand-held’ requirements, because they are capable of receiving data from other objects in the environment, at the cost of having a UWB receiver on the mobile device. However, they could only receive that feedback from another mobile device, due to the restriction on the use of UWB infrastructure. This prohibition is particularly restrictive for location systems which naturally involve interaction between mobile devices and devices placed at known and normally fixed points in space. Furthermore, it is not clear how they could meet the requirements of the indoor rules unless they are mains powered, which would significantly reduce their applicability.
Unidirectional UWB systems, such as the second described above, cannot satisfy the requirements of the ‘hand-held’ rules, because they cannot receive feedback on whether or not their signals have been detected, and they can only satisfy the requirements of the indoor rules if they are mains powered, again significantly reducing their applicability.
This invention aims to address at least some of the limitations associated with current systems.
According to one embodiment of the present invention, there is provided a portable location device for use in a location system, the device comprising a transmitter for transmitting ultra-wideband signals, a receiver for receiving non-ultra-wideband signals and a control unit coupled to the receiver for controlling the operation of the device in dependence on the received non-ultra-wideband signals.
Preferably, the control unit is arranged to control the rate at which the ultra-wideband signals are transmitted in dependence on the received non-ultra-wideband signals. The control unit may be arranged to control the channel over which the ultra-wideband signals are transmitted in dependence on the received non-ultra-wideband signals. The control unit may also be arranged to control the channel over which the non-ultra-wideband signals are received in dependence on the received non-ultra-wideband signals.
Preferably, the transmitter is arranged to transmit the ultra-wideband signals periodically and the control unit is arranged to stop the transmitter from transmitting those periodic ultra-wideband signals until a non-ultra-wideband signal of a predetermined type is received by the receiver.
The control unit may be arranged to start the transmitter transmitting the ultra-wideband signals when the receiver receives a non-ultra-wideband signal of a predetermined type.
Preferably, the ultra-wideband signals are suitable for allowing the location of the portable location device to be calculated. The ultra-wideband signals may be radio signals. The non-ultra-wideband signals may also be radio signals.
The transmitter may also be capable of transmitting non-ultra-wideband signals. The transmitter may also be capable of transmitting non-ultra-wideband signals containing an identifier that identifies the portable location device.
Preferably, the location device is arranged to periodically transmit an ultra-wideband signal and to transmit a non-ultra-wideband signal containing the identifier within a predetermined time of the transmission of the ultra-wideband signal. The predetermined time may be in a range from any of 0, 1, 2 or 5 seconds to any of 6, 10, 12 or 15 seconds.
According to a second aspect of the present invention, there is provided a location system comprising a plurality of receivers for receiving ultra-wideband signals from a portable device, a location unit for determining the location of the portable device in dependence on the ultra-wideband signals received by the receiver and a transmitter for transmitting control signals to the portable device in the form of non-ultra-wideband signals.
Preferably, the transmitter is arranged to periodically transmit the non-ultra-wideband signals. The transmitter may be arranged to broadcast or multicast the non-ultra-wideband signals to a plurality of portable devices. The transmitter may be arranged to transmit a non-ultra-wideband signal directed to a specific portable device in response to receiving a non-ultra-wideband signal from that device.
Preferably, the receiver is capable of receiving non-ultra-wideband signals.
Preferably, the location unit is arranged to attribute an ultra-wideband signal received at the receiver as being from a specific portable device in response to receiving a non-ultra-wideband signal identifying that device within a predetermined time of receiving the ultra-wideband signal.
The location reference device may comprise a handover unit, the handover unit being arranged to determine when handover of a tag from the location reference device to another location reference device is required and the transmitter being arranged to transmit a non-ultra-wideband signal indicating that handover is required to that tag if the handover unit determines that handover is required.
According to a third aspect of the present invention, there is provided a location system and a portable location device, the location system comprising a plurality of receivers for receiving ultra-wideband signals and a transmitter for transmitting non-ultra-wideband signals and the portable location device comprising a transmitter for transmitting ultra-wideband signals, a receiver for receiving non-ultra-wideband signals and a control unit coupled to the receiver for controlling the operation of the device in dependence on the received non-ultra-wideband signals.
Preferably, the location reference device comprises a location unit for determining the location of the portable location device in dependence on ultra-wideband signals received from the transmitter of the portable location device at least some of the receivers of the location system.
The transmitter of the location system may be arranged to periodically transmit non-ultra-wideband signals for reception at a receiver of the portable location device.
The receiver of the location system may be capable of receiving non-ultra-wideband signals and the transmitter of the portable location device may be capable of transmitting non-ultra-wideband signals.
Preferably, the location system comprises a plurality of base stations each comprising one of the receivers and a handover unit, the handover unit being arranged to determine when handover of the portable location device from one of the base stations to another base station is to be performed and the system is arranged so that when such a determination is made the transmitter of the location system transmits a non-ultra-wideband handover signal to the portable location device, and the control unit of the portable location device is arranged to change the channel on which it transmits ultra-wideband signals in response to reception of the handover signal by the receiver of the device.